U.S. Pat. No. 3,874,241 issued to Harland shows a pressure gauge in which the vent plug in accordance with the invention may be utilized. In the Harland gauge there is provided a pressure relief valve and a filling plug which is capable of being sealed and resealed by pushing in the plug to seal the gauge and pulling it out in order to vent the gauge. One of the problems found to be associated with the Harland type closures is that the separate venting and relieving assemblies may create problems for maintaining integrity against leaks in the gauge. Additionally, in many cases these assemblies may interfere with panel mounting applications due to the close fit required for the outer diameter of the gauge case.
U.S. Pat. No. 4,051,730 to Andrew et al discloses a pressure gauge having a blow out back assembly which includes a diaphragm and a resiliently firm back cover plate. The cover plate is bolted to the back side of the pressure gauge and apparently serves to hold the diaphragm in sealed contact with the inside of the pressure gauge.
U.S. Pat. No. 4,449,412 to Fallon et al. discloses a pressure relief valve means which includes a diaphragm biased by a coil spring. When the pressure exceeds the force exerted by the spring, the inlet is exposed to first and second relief passageways.
U.S. Pat. No. 5,012,678 to Buchanan discloses an overpressure relief safety plug for use with pressure gauges that includes a plug held in place by a strap and anchoring means fixing the plug to the exterior of the housing.
U.S. Pat. No. 4,064,890 to Collins et al. discloses a pressure regulator and safety relief valve where an elastomeric element is biased by a spring against an inlet port.
U.S. Pat. No. 2,522,007 to Willach discloses a pressure indicating and relieving device having an adjustable pressure relief means comprising a valve and longitudinal slot provided in the inner surface of a plug. Rotation of the valve adjusts fluid communication between a passageway and the longitudinal slot.
U.S. Pat. No. 3,701,284 to DeMeyer discloses a pressure gauge having a vented filler element provided in the upper portion of the gauge case. A rear closure plate is biased in place by a resilient element and provides a means for pressure relief when undue pressure builds up in the gauge housing.